Heretofore, attempts have been actively made to substitute glass glazing by transparent thermoplastic resin glazing so as to reduce the weight, improve the safety and achieve the utilization which is impossible with glass of the glazing. In the field of means of transport typified by automobiles, attempts to substitute glass glazing by polycarbonate resin glazing having high impact resistance in particular are actively under way. In the field of means of transport, it is an essential and urgent issue to reduce the weight of the glazing. Therefore, attempts to substitute glass are further accelerated. Although a large number of proposals are made, this substitution does not proceed well in the field of automobiles. One of the main reasons that this substitution does not proceed in spite of a strong demand for lightweight glazing is the high cost of resin glazing.
As means of absorbing the cost, there have been proposed a method in which the steps for obtaining resin glazing are simplified by making use of the excellent moldability of a resin and a method in which a design unable to be achieved with glass glazing is adopted. For example, a curved molded article is obtained by the injection molding of a resin without a thermoforming step (refer to Patent Document 1). There is further proposed a method of manufacturing a frame material to be attached to a car body and integrated with a resin window by means of a single molding machine, making use of a multi-color molding method (refer to Patent Document 2). However, to obtain resin glazing having little perspective distortion by the injection molding method, a large-sized mold having excellent surface accuracy must be used (refer to Patent Document 3). The production of this mold is costly, which is one of the causes of the high cost of the resin glazing. Especially an irregular curved surface cannot avoid finish by hand polishing which is apt to cause the fine surface undulation of the mold. As a result, the manufacture of a large-sized mold having an irregular curved surface and excellent surface accuracy requires a considerably high technique and a lot of time with the result that the mold becomes very expensive. That is, in the case of glazing for automobiles which differs in shape according to type, the amount of initial investment in the manufacture of the mold is huge, thereby further boosting the cost of the resin glazing.
Meanwhile, what is particularly notable in the injection molding method is that glazing having a high-grade surface which is superior to glass glazing can be manufactured when a mold having excellent surface accuracy is used and molding is carried out under suitable conditions. It is assumed that this is because the resin surface is pressed against the surface of the mold at a high pressure in the injection molding method, thereby transferring a shape having high accuracy precisely. It can be said that this is an advantage which is not obtained by glass glazing which is manufactured in an open system and thermally curved. If the surface is not curved, it can be finished at a significantly high surface accuracy only by NC machining, and this machining can achieve accuracy to such an extent that substantially no fine surface undulation is produced. That is, it is possible to provide a molded article having a considerably high surface accuracy at an extremely low cost if it is shaped like a plain sheet.
There is known a method for obtaining curved resin glazing by thermoforming an extruded sheet having a low production cost, and the applicant of the present application has proposed the curved resin glazing as a preferred member for construction machines (refer to Patent Document 4). Even in the field of automobiles, this method is one of well-known methods of manufacturing show cars and race cars. However, it cannot be said that this method is satisfactory for the formation of a high-grade design surface required for the glazing of a stock car.
Stated more specifically, even when a gray sheet whose defect is not seen is used in the thermoforming of an extruded sheet, a poor appearance such as a gear mark becomes obvious after thermoforming. It is assumed that the reason for this is that a polymer chain is heated at a temperature higher than the glass transition temperature to release its frozen strain, thereby releasing forced homogeneity to promote the heterogeneity of the remaining orientation strain. The gear mark of the extruded sheet is mainly caused by the nonuniform rotation of a cooling roll at the time of manufacture, and when the extruded sheet is especially inferior, it is also due to the pulse movement of a gear pump and the vibration of the whole apparatus. Currently, it is difficult to greatly reduce the nonuniform rotation of the giant cooling roll.
Since a nonuniform pattern is observed in a thermoformed extruded sheet according to an observation method or a light source even when a gray sheet which has a specified amount of perspective strain is used, the thermoformed extruded sheet may not be accepted in the field of automobiles which include a strong factor as articles of taste. The nonuniform pattern is a pattern in a state that a perspective image or a reflected image is seen like a flare when it is observed from a direction at an acute angle from the surface or under a relatively strong light source.
As described above, a thermoformed article having a high-grade design surface is hardly obtained by the molding of an extruded sheet, and a problem that high cost cannot be avoided by injection molding capable of forming a high-grade design surface is not solved yet. These problems are also described in Patent Document 5, for example.    (Patent Document 1) JP-A 2005-344006    (Patent Document 2) JP-A 2009-220554    (Patent Document 3) JP-A 2002-128909    (Patent Document 4) JP-A 2005-161652    (Patent Document 5) JP-A 10-119085